1. The Field of the Invention
The present invention generally relates to stators. More particularly, the present invention relates to a composite stator for use in rotationally driven systems, such as rotary anode x-ray generating devices.
2. The Related Technology
X-ray generating devices are extremely valuable tools that are used in a wide variety of applications, both industrial and medical. For example, such equipment is commonly employed in areas such as medical diagnostic examination, therapeutic radiology, semiconductor fabrication, and materials analysis.
Regardless of the applications in which they are employed, most x-ray generating devices operate in a similar fashion. X-rays are produced in such devices when electrons are emitted, accelerated, then impinged upon a material of a particular composition. This process typically takes place within an x-ray tube located in the x-ray generating device. The x-ray tube generally includes a vacuum enclosure comprising a cylindrical top section having a specified diameter, and a bottom cylindrical section having a smaller diameter. The top end bottom sections are hermitically sealed together to enable a vacuum to be maintained therein.
The vacuum enclosure has disposed therein a cathode and an anode. The cathode includes a filament that, when heated via an electrical current passing through it, emits a stream of electrons. The anode typically comprises a graphite substrate upon which is disposed a heavy metallic target surface that is oriented to receive the electrons emitted by the cathode. Though some x-ray tube anodes are stationary, many are rotatably supported within the vacuum enclosure by a rotor assembly.
A rotor assembly typically comprises a rotor shaft, a rotor hub and sleeve, and a bearing assembly. One end of the rotor shaft rotatably supports the rotary anode, while the other end is attached to the rotor hub and sleeve. The hub interconnects the rotor shaft and the rotor sleeve with the bearing assembly, thereby enabling the shaft and sleeve to rotate. The rotor sleeve is rotationally and concentrically disposed about a substantial portion of the bearing assembly.
The rotor assembly and anode are rotated by a stator. The stator, which generally comprises a hollow cylindrical core and a plurality of integrated wire coil windings, is circumferentially disposed about a portion of the exterior of the vacuum enclosure such that it is proximate the rotor sleeve disposed within the vacuum enclosure. When energized by a single or multi-phase electric current, the coil windings of the stator induce rotation of the rotor sleeve by way of electromagnetic induction. In this way, the anode, being connected to the rotor sleeve via the rotor shaft is rotated during operation of the x-ray tube.
In order for the x-ray tube to produce x-rays, an electric current is supplied to the cathode filament of the x-ray tube, causing it to emit a stream of electrons by thermionic emission. A high voltage potential placed between the cathode and the anode causes the electrons in the electron stream to gain kinetic energy and accelerate toward the target surface located on the anode. Upon striking the target surface, many of the electrons convert their kinetic energy into electromagnetic radiation of very high frequency, i.e., x-rays. The specific frequency of the x-rays produced depends in large part on the type of material used to form the anode target surface. Target surface materials having high atomic numbers (“Z numbers”), such as tungsten or TZM (an alloy of titanium, zirconium, and molybdenum) are typically employed. Finally, the x-ray beam passes through a window defined in the vacuum enclosure and is directed to an x-ray subject, such as a medical patient.
One common variety of x-ray tube employing a rotary anode is known as a double-ended x-ray tube. Double-ended x-ray tubes create the high voltage potential necessary to accelerate the electrons produced by the cathode toward the anode by electrically biasing both the cathode and the anode with a high negative and high positive voltage, respectively. A typical high power, double-ended x-ray tubes may electrically bias the anode and cathode with a relative voltage as high as 150 kV or more during tube operation.
Because typical high power, double-ended x-ray tubes operate with such high voltages, the use of insulating structures supportably connecting the anode and cathode to the vacuum enclosure is necessary to electrically isolate them from the rest of the tube. These insulating structures are typically composed of an electrically insulative material, such as glass or ceramic, and must isolate the high voltage present at the anode and cathode so that the vacuum enclosure and other parts of the tube are maintained at low voltage ground potential.
In the case of the rotary anode, an anode insulator is disposed between the rotor assembly and the bottom section of the vacuum enclosure. In addition to structurally supporting the anode via the rotor assembly, the anode insulator electrically isolates it, as explained above. The anode insulator comprises a ceramic disk having a central bore in which a portion of the bearing assembly of the rotor assembly is disposed. The anode insulator is disposed at the lower end of the bottom section of the vacuum enclosure, thereby comprising the circular bottom surface of the enclosure.
Because of the high voltage that must be isolated, the anode insulator must comprise at least a minimum dimension in order for it to properly isolate the anode and the attached rotor assembly. For instance, in a 150 kV double ended x-ray tube, the ceramic disk comprising the anode insulator typically has a minimum outside diameter of about 3.5 inches. Accordingly, the outside diameter of the bottom section of the vacuum enclosure bottom section must be at least about 3.5 inches.
Unfortunately, the minimum size requirement of the disk-shaped anode insulator creates other problems. For example, during manufacture of the x-ray tube, the top and bottom sections of the vacuum enclosure are joined by a high heat brazing process involving temperatures exceeding 400° Celsius. Because a typical stator comprises components that are able to withstand temperatures only up to about 220° Celsius, the stator must be joined to the vacuum enclosure only after the top and bottom sections of the enclosure have been joined. The joining of the stator to the vacuum enclosure is typically accomplished by sliding the hollow cylindrical stator over the end of the bottom section of the vacuum enclosure until it is disposed adjacent the rotor sleeve of the rotor assembly, where it is then secured. However, in order to be able to clear the circular anode insulator, which comprises the bottom surface of the bottom section of the vacuum enclosure, the inside diameter of the hollow cylindrical stator must be greater than the outside diameter of the anode insulator. Thus, the inside diameter of the typical stator is limited by the outside diameter of the anode insulator.
This limitation on the inside diameter of the typical stator is problematic. In order to cause the rotor sleeve to spin, the inside diameter of the stator must be as close as possible to the outside diameter of the rotor sleeve in order for inductive coupling between the two components to occur. Because the inside diameter of a typical stator disposed in a high power, double-ended tube must be large in order to fit over the bottom section of the vacuum enclosure, it does not enable inductive coupling with a standard-sized rotor sleeve commonly found in x-ray tubes. Thus, rotor sleeves having larger diameters must be specially manufactured for use in such high power x-ray tubes in order to provide the stator-to-sleeve proximity required to rotate the rotor assembly and anode. This in turn translates into increased expense in manufacturing and assembling the x-ray tube.
In addition to the above, other problems are created by the large diameter anode insulator, rotor sleeve, and stator. For example, the junction between the top and bottom sections of the vacuum enclosure must define a larger diameter aperture in order to accommodate insertion of the enlarged rotor sleeve into the vacuum enclosure during assembly of the x-ray tube. Unfortunately, this also creates an enlarged direct thermal path through which heat from the anode may be directly radiated to portions of the rotor assembly and bearing assembly during tube operation. Because the bearing assembly is especially heat sensitive and may be easily damaged if subjected to excessive amounts of heat, the increased radiative heat transfer created by the enlarged vacuum enclosure aperture is especially problematic.
In light of the above, a need exists to avoid the problems created by the design of high power, double-ended x-ray tubes necessitated by the high voltages present therein. Specifically, a need exists for a stator that can enable the above-described problems to be overcome not only in x-ray tubes, but in other rotationally-driven apparatus.